The present invention generally relates to image processing, and more particularly to an image separator for use in a color image processing apparatus for segmenting a color image that has been read by an image reading apparatus into regions that contain black line patterns, colored line patterns, and colored halftone pictures.
In the digital color copiers, and the like, an optimum image processing is needed according to the type of the images that is going to be recorded. For example, a typical color image may include black line portion such as the edge part of black characters and lines, colored line portions such as the edge part of colored characters and lines, and colored halftone images such as color pictures and screened color photographs that contain a number of color dots. Thereby, it is necessary to apply different image processing techniques adaptively according to the type of images to maximize the quality of images that are reproduced. As a result of such an optimum image processing, one may able to reproduce the black line portions or black characters by a black ink while the colored line portions or colored characters by colored inks. Further, one can reproduce the colored halftone images without the problem of forming moire. In order to apply such an optimum image processing adaptively according to the type of images, it is necessary to discriminate and segment the original images into regions that contain respective types of images.
Conventionally, a color image forming apparatus is known that applies such a segmentation of the original color images according to the Japanese Laidopen Patent Application 63-240175. This conventional apparatus extracts the region containing bi-level black images and characters from the original color image on the document and reproduces the region thus extracted by the black ink. The apparatus uses, for the purpose of recognition, the YMC (Yellow, Magenta, Cyan) signals that are obtained by a color correction process of the usual RGB (Red, Green, Blue) signals that in turn are obtained by the color decomposition of the original color images. The discrimination about the type of the images is achieved by comparing the level of the Y, M and C signals against a predetermined threshold. More specifically, the image on the document is segmented into a number of regions, and in each region, the image is recognized as a bi-level black image when the sum of the level of the Y, M and C signals has a level exceeding the predetermined threshold.
When applying this conventional process against the halftone, color dot images, however, there arises a problem of erroneous recognition of the images. In such a halftone, color dot images, it should be noted that the color dots forming the images and bearing the colors of Y, M or C are generally overlapped with each other. In other words, there occurs a case relatively often that the sum of the level of the Y, M and C signals obtained by reading the original image shows a level that exceeds the predetermined threshold, even though the image is not a black, bi-level image. Thus, it will be understood that the discrimination of whether a given color image represents the color picture pattern or bi-level black pattern is not a simple matter, and there is a substantial risk that one erroneously discriminates the color picture pattern as black, bi-level pattern. When such an erroneous recognition occurs, black and white images appear in the reproduced color image and the quality of the reproduced image is significantly deteriorated rather than improved. Thus, the foregoing conventional apparatus has suffered from the problem of erroneous segmentation of the image and deterioration in the quality of the reproduced color images.